Mems devices having support structures with substantially vertical sidewalls and methods for fabricating the same

ABSTRACT

Embodiments of MEMS devices include support structures having substantially vertical sidewalls. Certain support structures are formed through deposition of self-planarizing materials or via a plating process. Other support structures are formed via a spacer etch. Other MEMS devices include support structures at least partially underlying a movable layer, where the portions of the support structures underlying the movable layer include a convex sidewall. In further embodiments, a portion of the support structure extends through an aperture in the movable layer and over at least a portion of the movable layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/506,600, filed Aug. 18, 2006, now issued as U.S. Pat. No. 7,704,773,which claims priority under 35 U.S.C. §119(e) to U.S. ProvisionalApplication Ser. No. 60/710,019, filed Aug. 19, 2005, each of which ishereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and/or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. As used herein, theterm interferometric modulator or interferometric light modulator refersto a device that selectively absorbs and/or reflects light using theprinciples of optical interference. In certain embodiments, aninterferometric modulator may comprise a pair of conductive plates, oneor both of which may be transparent and/or reflective in whole or partand capable of relative motion upon application of an appropriateelectrical signal. In a particular embodiment, one plate may comprise astationary layer deposited on a substrate and the other plate maycomprise a metallic membrane separated from the stationary layer by anair gap. As described herein in more detail, the position of one platein relation to another can change the optical interference of lightincident on the interferometric modulator. Such devices have a widerange of applications, and it would be beneficial in the art to utilizeand/or modify the characteristics of these types of devices so thattheir features can be exploited in improving existing products andcreating new products that have not yet been developed.

SUMMARY OF THE INVENTION

In one embodiment, a method of fabricating a MEMS device is provided,the method including providing a substrate, depositing an electrodelayer over the substrate, depositing a sacrificial layer over theelectrode layer, patterning the sacrificial layer to form an aperture,depositing a layer of inorganic self-planarizing material over thesacrificial layer, such that it fills the aperture, etching back thelayer of self-planarizing material to a level at or below the uppersurface of the sacrificial layer to form a support structure, anddepositing a movable layer over the support structure.

In another embodiment, a method of fabricating a MEMS device isprovided, the method including providing a substrate, depositing anelectrode layer over the substrate, depositing a sacrificial layer overthe electrode layer, patterning the sacrificial layer to define anaperture, forming a metallic seed layer, where the metallic seed layerdoes not extend over unpatterned portions of the sacrificial layer,forming a support structure within the aperture via a plating process,and depositing a movable layer over the support structure.

In another embodiment, a method of fabricating a MEMS device isprovided, the method including providing a substrate, depositing anelectrode layer over the substrate, depositing a sacrificial layer overthe electrode layer, patterning the sacrificial layer to define anaperture having a substantially vertical sidewall, depositing a layer ofconformal support material over the vertical sidewall and over an uppersurface of the sacrificial layer, performing a directional etch to forma spacer structure located within the aperture and against thesubstantially vertical sidewall, where the directional etch removes thesupport material overlying the upper surface of the sacrificial layer,and depositing a movable layer over the support structure.

In another embodiment, a method of fabricating a MEMS device isprovided, the method including providing a substrate, depositing anelectrode layer over the substrate, depositing a sacrificial layer overthe electrode layer, depositing a movable layer over the sacrificiallayer, patterning the movable layer to form an aperture extendingthrough the movable layer, thereby exposing a portion of the sacrificallayer, etching the exposed portion of the sacrificial layer to form acavity extending through the sacrificial layer and undercutting aportion of the movable layer, and depositing a layer of self-planarizingsupport material to fill the cavity.

In another embodiment, a MEMS device is provided, including a substrate,an electrode layer located over the substrate, a movable layer locatedover the electrode layer, where the movable layer is generally spacedapart from the electrode layer by an air gap, and an inorganic supportstructure underlying the movable layer, where the inorganic supportstructure includes a substantially vertical sidewall, and where theinorganic support structure is spaced apart from the substrate by atleast one intermediate layer.

In another embodiment, a MEMS device is provided, including a substrate,an electrode layer located over the substrate, a movable layer locatedover the electrode layer, where the movable layer is generally spacedapart from the electrode layer by an air gap, the movable layerincluding an aperture extending through the movable layer, and a supportstructure located at least partially beneath the aperture in the movablelayer, the support structure including a convex sidewall portion locatedunderneath the movable layer.

In another embodiment, a MEMS device, is provided, including first meansfor electrically conducting, second means for electrically conducting,and means for supporting the second conducting means over the firstconducting means, where the second conducting means is movable relativeto the first conducting means in response to generating electrostaticpotential between the first and second conducting means, and where thesupporting means extend through an aperture in the second conductingmeans and enclose at least a portion of the second conducting means.

In another embodiment, a method of manufacturing a MEMS device isprovided, the method including forming a lower sacrificial layer over asubstrate, forming a movable layer over the first sacrificial layer,forming an upper sacrificial layer over the movable layer, where thethickness of the second sacrificial layer is between 30 and 500angstroms, forming a rigid ceiling layer over the second sacrificiallayer, and forming a support structure which provides support to boththe movable layer and the rigid ceiling layer.

In another embodiment, a MEMS device is provided, including a movablelayer spaced apart from a substrate by a lower air gap, a rigid ceilinglayer spaced apart from the movable layer by an upper air gap, where theheight of the upper air gap is between 30 and 500 angstroms, and asupport structure which provides support to both the movable layer andthe rigid ceiling layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIG. 5A illustrates one exemplary frame of display data in the 3×3interferometric modulator display of FIG. 2.

FIG. 5B illustrates one exemplary timing diagram for row and columnsignals that may be used to write the frame of FIG. 5A.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa visual display device comprising a plurality of interferometricmodulators.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D is a cross section of yet another alternative embodiment of aninterferometric modulator.

FIG. 7E is a cross section of an additional alternative embodiment of aninterferometric modulator.

FIG. 8 is a top plan view of an array of MEMS elements in which theindividual elements comprise support structures.

FIGS. 9A-9I are schematic cross-sections illustrating steps in a methodfor fabricating a MEMS device having a support structure formed from aself-planarizing material.

FIG. 10 is a schematic cross-sections illustrating a step in analertnate method for fabricating a MEMS device having a supportstructure formed from a self-planarizing material.

FIGS. 11A-11F are schematic cross-sections illustrating steps in amethod for fabricating a MEMS device having an electrode which ispartially separated from a mechanical layer.

FIGS. 12A-12D are schematic cross-sections illustrating steps in amethod for fabricating a MEMS device having a support structure whichencloses at least a portion of a movable layer.

FIG. 13 is a schematic cross-section illustrating a step in a method forfabricating a MEMS device having a support structure which includes anaperture extending through the support structure.

FIGS. 14A-14B are schematic cross-sections illustrating steps in analternate method for fabricating a MEMS device having a supportstructure formed by an electroplating process.

FIGS. 15A-15C are schematic cross-sections illustrating steps in amethod for fabricating a MEMS device having spacers formed by a spaceretch.

FIGS. 16A-16C are schematic cross-sections illustrating steps in amethod for fabricating a MEMS device having an overlying rigid ceilingmember.

FIGS. 17A-17B are schematic cross-sections illustrating steps in analternate method for fabricating a MEMS device having an overlying rigidceiling member.

FIG. 18 is a schematic cross-section illustrating a step in an alternatemethod for fabricating a MEMS device having an overlying rigid ceilingmember.

FIG. 19 is a schematic cross-section illustrating a step in an alternatemethod for fabricating a MEMS device having a support structure formedby an electroplating process

FIGS. 20A-20C are schematic cross-sections illustrating steps in analternate method for fabricating a MEMS device having a supportstructure formed by an electroplating process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theembodiments may be implemented in any device that is configured todisplay an image, whether in motion (e.g., video) or stationary (e.g.,still image), and whether textual or pictorial. More particularly, it iscontemplated that the embodiments may be implemented in or associatedwith a variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

While it is desirable to provide additional support to movable layers inMEMS elements in order to ensure the desired spacing is maintainedbetween the movable layer and other components, the inclusion of suchsupport structures inhibits the motion of the movable layer in the areasurrounding the support structures, and may have an adverse effect onthe performance of the MEMS device, effectively reducing the active areaof the

MEMS device. It is thus desirable to minimize the footprint of thesesupport structures while providing the desired level of support. Incertain embodiments, this can be achieved through the use of supportstructures having substantially vertical sidewalls. In one embodiment,the fabrication of such support structures can be achieved through theuse of a self-planarizing material. In further embodiments, thesesupport structures may extend through an aperture in the movable layerand enclose a portion of the movable layer. In alternate embodiments,directional etches or electroplating techniques can be used to providesuch support structures.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as therelaxed position, the movable reflective layer is positioned at arelatively large distance from a fixed partially reflective layer. Inthe second position, referred to herein as the actuated position, themovable reflective layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable reflective layer 14 a isillustrated in a relaxed position at a predetermined distance from anoptical stack 16 a, which includes a partially reflective layer. In theinterferometric modulator 12 b on the right, the movable reflectivelayer 14 b is illustrated in an actuated position adjacent to theoptical stack 16 b.

The optical stacks 16 a and 16 b (collectively referred to as opticalstack 16), as referenced herein, typically comprise several fusedlayers, which can include an electrode layer, such as indium tin oxide(ITO), a partially reflective layer, such as chromium, and a transparentdielectric. The optical stack 16 is thus electrically conductive,partially transparent, and partially reflective, and may be fabricated,for example, by depositing one or more of the above layers onto atransparent substrate 20. The partially reflective layer can be formedfrom a variety of materials that are partially reflective such asvarious metals, semiconductors, and dielectrics. The partiallyreflective layer can be formed of one or more layers of materials, andeach of the layers can be formed of a single material or a combinationof materials.

In some embodiments, the layers of the optical stack 16 are patternedinto parallel strips, and may form row electrodes in a display device asdescribed further below. The movable reflective layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes of 16 a, 16 b) deposited on topof posts 18 and an intervening sacrificial material deposited betweenthe posts 18. When the sacrificial material is etched away, the movablereflective layers 14 a, 14 b are separated from the optical stacks 16 a,16 b by a defined gap 19. A highly conductive and reflective materialsuch as aluminum may be used for the reflective layers 14, and thesestrips may form column electrodes in a display device.

With no applied voltage, the cavity 19 remains between the movablereflective layer 14 a and optical stack 16 a, with the movablereflective layer 14 a in a mechanically relaxed state, as illustrated bythe pixel 12 a in FIG. 1. However, when a potential difference isapplied to a selected row and column, the capacitor formed at theintersection of the row and column electrodes at the corresponding pixelbecomes charged, and electrostatic forces pull the electrodes together.If the voltage is high enough; the movable reflective layer 14 isdeformed and is forced against the optical stack 16. A dielectric layer(not illustrated in this Figure) within the optical stack 16 may preventshorting and control the separation distance between layers 14 and 16,as illustrated by pixel 12 b on the right in FIG. 1. The behavior is thesame regardless of the polarity of the applied potential difference. Inthis way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5B illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate aspects of the invention. In theexemplary embodiment, the electronic device includes a processor 21which may be any general purpose single- or multi-chip microprocessorsuch as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®,Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any specialpurpose microprocessor such as a digital signal processor,microcontroller, or a programmable gate array. As is conventional in theart, the processor 21 may be configured to execute one or more softwaremodules. In addition to executing an operating system, the processor maybe configured to execute one or more software applications, including aweb browser, a telephone application, an email program, or any othersoftware application.

In one embodiment, the processor 21 is also configured to communicatewith an array driver 22. In one embodiment, the array driver 22 includesa row driver circuit 24 and a column driver circuit 26 that providesignals to a display array or panel 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the relaxed state to the actuated state.However, when the voltage is reduced from that value, the movable layermaintains its state as the voltage drops back below 10 volts. In theexemplary embodiment of FIG. 3, the movable layer does not relaxcompletely until the voltage drops below 2 volts. Thus, there exists awindow of applied voltage, about 3 to 7 V in the example illustrated inFIG. 3, within which the device is stable in either the relaxed oractuated state. This is referred to herein as the “hysteresis window” or“stability window.” For a display array having the hysteresischaracteristics of FIG. 3, the row/column actuation protocol can bedesigned such that during row strobing, pixels in the strobed row thatare to be actuated are exposed to a voltage difference of about 10volts, and pixels that are to be relaxed are exposed to a voltagedifference of close to zero volts. After the strobe, the pixels areexposed to a steady state voltage difference of about 5 volts such thatthey remain in whatever state the row strobe put them in. After beingwritten, each pixel sees a potential difference within the “stabilitywindow” of 3-7 volts in this example. This feature makes the pixeldesign illustrated in FIG. 1 stable under the same applied voltageconditions in either an actuated or relaxed pre-existing state. Sinceeach pixel of the interferometric modulator, whether in the actuated orrelaxed state, is essentially a capacitor formed by the fixed and movingreflective layers, this stable state can be held at a voltage within thehysteresis window with almost no power dissipation. Essentially nocurrent flows into the pixel if the applied potential is fixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4, 5A, and 5B illustrate one possible actuation protocol forcreating a display frame on the 3×3 array of FIG. 2. FIG. 4 illustratesa possible set of column and row voltage levels that may be used forpixels exhibiting the hysteresis curves of

FIG. 3. In the FIG. 4 embodiment, actuating a pixel involves setting theappropriate column to −V_(bias), and the appropriate row to +ΔV, whichmay correspond to −5 volts and +5 volts, respectively Relaxing the pixelis accomplished by setting the appropriate column to +V_(bias), and theappropriate row to the same +ΔV, producing a zero volt potentialdifference across the pixel. In those rows where the row voltage is heldat zero volts, the pixels are stable in whatever state they wereoriginally in, regardless of whether the column is at +V_(bias), or−V_(bias). As is also illustrated in FIG. 4, it will be appreciated thatvoltages of opposite polarity than those described above can be used,e.g., actuating a pixel can involve setting the appropriate column to+V_(bias), and the appropriate row to −ΔV. In this embodiment, releasingthe pixel is accomplished by setting the appropriate column to−V_(bias), and the appropriate row to the same −ΔV, producing a zerovolt potential difference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or relaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thesystems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 45, an input device 48, and a microphone 46. The housing41 is generally formed from any of a variety of manufacturing processesas are well known to those of skill in the art, including injectionmolding and vacuum forming. In addition, the housing 41 may be made fromany of a variety of materials, including, but not limited to, plastic,metal, glass, rubber, and ceramic, or a combination thereof. In oneembodiment, the housing 41 includes removable portions (not shown) thatmay be interchanged with other removable portions of different color, orcontaining different logos, pictures, or symbols.

The display 30 of the exemplary display device 40 may be any of avariety of displays, including a bi-stable display, as described herein.In other embodiments, the display 30 includes a flat-panel display, suchas plasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device, as is wellknown to those of skill in the art. However, for purposes of describingthe present embodiment, the display 30 includes an interferometricmodulator display, as described herein.

The components of one embodiment of the exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43, which is coupled to a transceiver 47. Thetransceiver 47 is connected to a processor 21, which is connected toconditioning hardware 52. The conditioning hardware 52 may be configuredto condition a signal (e.g., filter a signal). The conditioning hardware52 is connected to a speaker 45 and a microphone 46. The processor 21 isalso connected to an input device 48 and a driver controller 29. Thedriver controller 29 is coupled to a frame buffer 28 and to an arraydriver 22, which in turn is coupled to a display array 30. A powersupply 50 provides power to all components as required by the particularexemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one or moredevices over a network. In one embodiment, the network interface 27 mayalso have some processing capabilities to relieve requirements of theprocessor 21. The antenna 43 is any antenna known to those of skill inthe art for transmitting and receiving signals. In one embodiment, theantenna transmits and receives RF signals according to the IEEE 802.11standard, including IEEE 802.11(a), (b), or (g). In another embodiment,the antenna transmits and receives RF signals according to the BLUETOOTHstandard. In the case of a cellular telephone, the antenna is designedto receive CDMA, GSM, AMPS, or other known signals that are used tocommunicate within a wireless cell phone network. The transceiver 47pre-processes the signals received from the antenna 43 so that they maybe received by and further manipulated by the processor 21. Thetransceiver 47 also processes signals received from the processor 21 sothat they may be transmitted from the exemplary display device 40 viathe antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, the network interface27 can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe memory device such as a digital video disc (DVD) or a hard-disc drivethat contains image data, or a software module that generates imagedata.

The processor 21 generally controls the overall operation of theexemplary display device 40. The processor 21 receives data, such ascompressed image data from the network interface 27 or an image source,and processes the data into raw image data or into a format that isreadily processed into raw image data. The processor 21 then sends theprocessed data to the driver controller 29 or to the frame buffer 28 forstorage. Raw data typically refers to the information that identifiesthe image characteristics at each location within an image. For example,such image characteristics can include color, saturation, and gray-scalelevel.

In one embodiment, the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40. Theconditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 45, and for receiving signals fromthe microphone 46. The conditioning hardware 52 may be discretecomponents within the exemplary display device 40, or may beincorporated within the processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, the array driver 22, andthe display array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, the driver controller29 is a conventional display controller or a bi-stable displaycontroller (e.g., an interferometric modulator controller). In anotherembodiment, the array driver 22 is a conventional driver or a bi-stabledisplay driver (e.g., an interferometric modulator display). In oneembodiment, a driver controller 29 is integrated with the array driver22. Such an embodiment is common in highly integrated systems such ascellular phones, watches, and other small area displays. In yet anotherembodiment, the display array 30 is a typical display array or abi-stable display array (e.g., a display including an array ofinterferometric modulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, the input device 48includes a keypad, such as a QWERTY keyboard or a telephone keypad, abutton, a switch, a touch-sensitive screen, or a pressure- orheat-sensitive membrane. In one embodiment, the microphone 46 is aninput device for the exemplary display device 40. When the microphone 46is used to input data to the device, voice commands may be provided by auser for controlling operations of the exemplary display device 40.

The power supply 50 can include a variety of energy storage devices asare well known in the art. For example, in one embodiment, the powersupply 50 is a rechargeable battery, such as a nickel-cadmium battery ora lithium ion battery. In another embodiment, the power supply 50 is arenewable energy source, a capacitor, or a solar cell including aplastic solar cell, and solar-cell paint. In another embodiment, thepower supply 50 is configured to receive power from a wall outlet.

In some embodiments, control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some embodiments, controlprogrammability resides in the array driver 22. Those of skill in theart will recognize that the above-described optimizations may beimplemented in any number of hardware and/or software components and invarious configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7E illustrate five different embodiments of themovable reflective layer 14 and its supporting structures. FIG. 7A is across section of the embodiment of FIG. 1, where a strip of metalmaterial 14 is deposited on orthogonally extending supports 18. In FIG.7B, the moveable reflective layer 14 is attached to supports 18 at thecorners only, on tethers 32. In FIG. 7C, the moveable reflective layer14 is suspended from a deformable layer 34, which may comprise aflexible metal. The deformable layer 34 connects, directly orindirectly, to the substrate 20 around the perimeter of the deformablelayer 34. These connections are herein referred to as supportstructures, which can take the form of isolated pillars or posts and/orcontinuous walls or rails. The embodiment illustrated in FIG. 7D hassupport structures 18 that include support plugs 42 upon which thedeformable layer 34 rests. The movable reflective layer 14 remainssuspended over the cavity, as in FIGS. 7A-7C, but the deformable layer34 does not form the support posts by filling holes between thedeformable layer 34 and the optical stack 16. Rather, the support posts18 are formed of a planarization material, which is used to form thesupport post plugs 42. The embodiment illustrated in FIG. 7E is based onthe embodiment shown in FIG. 7D, but may also be adapted to work withany of the embodiments illustrated in FIGS. 7A-7C, as well as additionalembodiments not shown. In the embodiment shown in FIG. 7E, an extralayer of metal or other conductive material has been used to form a busstructure 44. This allows signal routing along the back of theinterferometric modulators, eliminating a number of electrodes that mayotherwise have had to be formed on the substrate 20.

In embodiments such as those shown in FIG. 7, the interferometricmodulators function as direct-view devices, in which images are viewedfrom the front side of the transparent substrate 20, the side oppositeto that upon which the modulator is arranged. In these embodiments, thereflective layer 14 optically shields the portions of theinterferometric modulator on the side of the reflective layer oppositethe substrate 20, including the deformable layer 34. This allows theshielded areas to be configured and operated upon without negativelyaffecting the image quality. Such shielding allows the bus structure 44in FIG. 7E, which provides the ability to separate the opticalproperties of the modulator from the electromechanical properties of themodulator, such as addressing and the movements that result from thataddressing. This separable modulator architecture allows the structuraldesign and materials used for the electromechanical aspects and theoptical aspects of the modulator to be selected and to functionindependently of each other. Moreover, the embodiments shown in FIGS.7C-7E have additional benefits deriving from the decoupling of theoptical properties of the reflective layer 14 from its mechanicalproperties, which are carried out by the deformable layer 34. Thisallows the structural design and materials used for the reflective layer14 to be optimized with respect to the optical properties, and thestructural design and materials used for the deformable layer 34 to beoptimized with respect to desired mechanical properties.

In certain embodiments, it may be desirable to provide additionalsupport to a movable layer such as the movable reflective layer 14illustrated in FIG. 7A, or the combination of mechanical layer 34 andmovable reflective layer 14 of FIGS. 7C-7E. In optical MEMS, such as aninterferometric modulator, the movable layer may comprise a reflectivesublayer and a mechanical sublayer, as will be discussed in greaterdetail below. Such support may be provided by a series of supportstructures which may be located along the edges of an individualmodulator element and/or in the interior of such an element. In variousembodiments, these support structures may be located either over orunderneath a movable layer. In alternate embodiments, support structuresmay extend through an aperture formed in the mechanical layer, such thatsupport is provided from both above and below the mechanical layer. Asused herein, the term “rivet” generally refers to a patterned layeroverlying a mechanical layer in a MEMS device, usually in a recess ordepression in the post or support region, to lend mechanical support forthe mechanical layer. Preferably, though not always, the rivet includeswings overlying an upper surface of the mechanical layer to addstability and predictability to the mechanical layer's movement.Similarly, support structures underlying a mechanical layer in a MEMSdevice to lend mechanical support for the mechanical layer are generallyreferred to herein as support “posts.” In many of the embodimentsherein, the preferred materials are inorganic for stability relative toorganic resist materials.

An exemplary layout of such support structures is shown in FIG. 8, whichdepicts an array of MEMS elements. In certain embodiments, the array maycomprise an array of interferometric modulators, but in alternateembodiments, the MEMS elements may comprise any MEMS device having amovable layer. It can be seen that support structures 62 are locatedboth along the edges of a movable layer 66 and in the interior of a MEMSelement, in this example an interferometric modulator element 60.Certain support structures may comprise rail structures 64, which extendacross the gap 65 between two adjacent movable layers 66. It can be seenthat movable layer 66 comprises a strip of deformable material extendingthrough multiple adjacent elements 60 within the same column. The railstructure 64 run parallel with lowe electrodes, which define rowscrossing the upper electrodes defined by the strips of the movable layer66. The support structures 62 serve to stiffen the movable layer 66within the elements or pixels 60.

Advantageously, these support structures 62 are made small relative tothe surrounding area of the modulator element 60. As the support postsconstrain deflection of the movable layer 66 and may generally beopaque, the area underneath and immediately surrounding the supportstructures 62 is not usable as active area in a display, as the movablelayer in those areas is not movable to a fully actuated position (e.g.,one in which a portion of the lower surface of the movable layer 14 ofFIG. 7A is in contact with the upper surface of the optical stack 16).Because this may result in undesirable optical effects in the areassurrounding the post, a dark or “black” mask layer may advantageously beprovided between the support structures and the viewer to avoidexcessive reflection in these regions that may wash out the intendedcolor.

In addition, as the area immediately surrounding the support structureis not useable as an active area in the display, it is desirable tominimize the size of the support structures to the extent possible whilestill providing the desired amount of support. In certain embodiments,the formation of these support structures involves the deposition oflayers over tapered underlying layers, so as to permit conformaldeposition of layers which form the support structure, resulting insupport structures having tapered sidewall portions. While suchembodiments ensure the conformal deposition of the layers which form thesupport structure, the tapered sidewall portions may make the supportstructure larger than desirable. However, embodiments of supportstructures which need not include a tapered sidewall portion arediscussed below.

In certain embodiments, a spin-on material, such as a spin-on glass orNissan Hardcoat, can be used to form various support structures,including rivet structures and inorganic post structures. In oneembodiment, described with respect to FIGS. 9A-9J, spin-on glass orother self-planarizing material (other than resist) is used to form poststructures.

In FIG. 9A, it can be seen that a transparent or light-transmissivesubstrate 70 is provided, which may comprise, for example, glass or atransparent polymeric material. A conductive layer 72, which maycomprise indium-tin-oxide (ITO), is then deposited over the transparentsubstrate and a partially reflective layer 74, which may comprisechromium, is deposited over the conductive layer 72. Although in oneembodiment conductive layer 72 may comprise ITO, and may be referred toas such at various points in the below specification, it will beunderstood that the layer 72 may comprise any suitable conductivematerial, and need not be transparent for non-optical MEMS structures.Similarly, although sometimes referred to as a chromium layer, partiallyreflective layer 74 may comprise any suitable partially reflectivelayer, and may be omitted for non-optical MEMS structures.

The conductive layer 72 and partially reflective layer 74 are thenpatterned and etched to form bottom electrodes, also referred to as rowelectrodes, which run cross-wise (e.g., perpendicular) to the movablelayer 66 of FIG. 8 and which will be used to address a row of MEMSelements. In certain embodiments, the conductive and partiallyreflective layers 72 and 74 may advantageously also be patterned andetched to remove the ITO and chromium underlying the areas where thesupport post structures will be located, forming apertures 76 asdepicted in FIG. 9B. This patterning and etching is preferably done bythe same process which forms the row electrodes. The removal of ITO andchromium (or other conductive materials) underlying the supportstructures helps to minimize the risk of shorting between an overlyingconductive layer, such as the movable layer, and the bottom electrode.Thus, FIG. 9B and the subsequent figures depict a cross-section of acontinuous row electrode formed by layers 72 and 74, in which isolatedapertures 76 have been etched, taken along a line extending throughthose apertures. In other embodiments in which the conductive layer 72and partially reflective layer 74 are not etched to form apertures 76, adielectric layer, discussed below, may provide sufficient protectionagainst shorting between the bottom electrode and the movable layer.

The conductive layer 72 and partially reflective layer 74 may bepatterned via photolithography and etched via, for example, commerciallyavailable wet etches. Chromium wet etches include solutions of aceticacid (C₂H₄O₂) and cerium ammonium nitrate [Ce(NH₄)₂(NO₃)₆]. ITO wetetches include HCl, a mixture of HCl and HNO₃, or a mixture ofFeCl₃/HCl/DI in a 75%/3%/22% ratio and H₂O. Once the apertures 76 havebeen formed, a dielectric layer 78 is deposited over the conductive andpartially reflective layers 72 and 74, as seen in FIG. 9C, forming theoptical stack 16. In certain embodiments, the dielectric layer maycomprise SiO₂ or SiN_(x), although a wide variety of suitable materialsmay be used.

A variety of methods can be used to perform the patterning and etchingprocesses discussed with respect to the various embodiments disclosedherein. The etches used may be either a dry etch or a wet etch, and maybe isotropic or anisotropic. Suitable dry etches include, but are notlimited to: SF₆/O₂, CHF₃/O₂, SF₂/O₂, CF₄/O₂, and NF₃/O₂. Generally,these etches are suitable for etching one or more of SiO_(x), SiN_(x),SiO_(x)N_(y), spin-on glass, Nissan™ hard coat, and TaO_(x), but othermaterials may also be etched by this process. Materials which areresistant to one or more of these etches, and may thus be used as etchbarrier layers, include but are not limited to Al, Cr, Ni, and Al₂O₃. Inaddition, wet etches including but not limited to PAD etches, BHF, KOH,and phosphoric acid may be utilized in the processes described herein,and may generally be used to etch metallic materials. Generally, theseetches may be isotropic, but can be made anisotropic through the use ofa reactive ion etch (RIE), by ionizing the etch chemicals and shootingthe ions at the substrate. The patterning may comprise the deposition ofa photoresist (PR) layer (either positive or negative photoresist),which is then used to form a mask. Alternately, a hard mask can beutilized. In some embodiments, the hard mask may comprise metal orSiN_(x), but it will be understood that the composition of the hard maskmay depend on the underlying materials to be etched and the selectivityof the etch to be used. In The hard mask is typically patterned using aPR layer, which is then removed, and the hard mask is used as a mask toetch an underlying layer. The use of a hard mask may be particularlyadvantageous when a wet etch is being used, or whenever processingthrough a mask under conditions that a PR mask cannot handle (such as athigh temperatures, or when using an oxygen-based etch). Alternatemethods of removing layers may also be utilized, such as an ashing etchor lift-off processes.

The thickness and positioning of the layers forming the optical stack 16determines the color reflected by the interferometric modulator elementwhen the element is actuated (collapsed), bringing the movable layer 66into contact with the optical stack 16. In certain embodiments, theoptical stack is configured such that the interferometric modulatorelement reflects substantially no visible light (appears black) when themovable layer is in an actuated position. Typically, the thickness ofthe dielectric layer 78 is about 450 A, although it will be understoodthat the desired thickness of the dielectric layer 78 will vary based onthe refractive index of the material and the desired color reflected bythe interferometric modulator in a collapsed state. While illustratedfor simplicity as planar (which can be achieved if the dielectric layer78 is a spin-on glass), the dielectric layer 78 is typically conformalover the patterned lower electrode formed from layers 72 and 74.

As seen in FIG. 9D, a layer 82 of sacrificial material is then depositedover the dielectric layer 78. In certain embodiments, this sacrificiallayer 82 is formed from a material which is etchable by fluorine-basedetchants, particularly XeF₂. For example, the sacrificial layer 82 maybe formed from molybdenum or amorphous silicon (a-Si). In otherembodiments, the sacrificial layer may comprise tantalum or tungsten.Other materials which are usable as sacrificial materials includesilicon nitride, certain oxides, and organic materials. The thickness ofthe deposited sacrificial layer 82 will determine the distance betweenthe optical stack 16 and the movable layer 66, thus defining thedimensions of the interferometric gap 19 (see FIG. 7A). As the height ofthe gap 19 determines the color reflected by the interferometricmodulator element when in an unactuated position, the thickness of thesacrificial layer 82 will vary depending on the desired characteristicsof the interferometric modulator. For instance, in an embodiment inwhich a modulator element that reflects green in the unactuated positionis formed, the thickness of the sacrificial layer 82 may be roughly 2000A. In further embodiments, the sacrificial layer may have multiplethicknesses across an array of MEMS devices, such as in a multicolordisplay system where different interferometric gap sizes are used toproduce different colors.

In FIG. 9E, it can be seen that the sacrificial layer 82 has beenpatterned and etched to form apertures 86. The apertures 86 overlie theapertures 76 cut into the layers 72 and 74 of ITO and chromium. Theseapertures 86 may be formed by masking the sacrificial layer, usingphotolithography, and then performing an etch to remove portions of thesacrificial material. Preferably a dry, directional etch is performed toobtain the near-vertical sidewalls shown. Preferably, the sidewallsslope less than about ±10° relative to vertical. Suitable dry etchesinclude, but are not limited to, SF₆, CF₄, Cl₂, or any mixture of thesegases with O₂ or a noble gas such as He or Ar.

As can be seen in FIG. 9F, a layer 110 of spin-on material is thendeposited over the patterned sacrificial layer 82, filling the apertures86. As noted above, the edges of these apertures are substantiallyvertical rather than tapered, as the spin-on material will fill theapertures 86 as a result of the spinning process, which causes thespin-on material to flow to fill such apertures. The deposition ofspin-on glass or other self-planarizing materials can be done in avariety of ways, including but not limited to exposure to a liquidprecursor, spray deposition, ink jet deposition, extrusion of thespin-on material, application via a roller coater, and screen printing.The materials used in the formation of the support structure arepreferably inorganic (e.g., SOG, which is a form of silicon oxide) forbetter stability relative to photoresist.

In certain embodiments, as can be seen in FIG. 9F, the spin-on layer 110extends above the sacrificial layer 82. In those embodiments, thespin-on layer 110 may be etched back such that the layer 110 only fillsthe apertures 86, and does not extend over the remaining sacrificialmaterial 82. In FIG. 9G, it can be seen that the layer has been blanketetched back without the need for a mask, forming inorganic posts 112 ofthe spin-on material. In this embodiment, the upper surface of theseinorganic posts 112 is substantially coplanar with, or slightly below(e.g., less than 5% of the height of the sacrificial layer) the uppersurface of the sacrificial layer 82.

In FIG. 9H, it can be seen that the components which will form themovable layer 66 (see, e.g., moveable reflective layer 14 in FIG. 7A)are then deposited over the patterned sacrificial layer 82 and posts 112In the embodiment of FIG. 9H, a highly reflective layer 90, alsoreferred to as a mirror or mirror layer, is deposited first, followed bya mechanical layer 92. The highly reflective layer 90 may be formed froma specular metal, such as aluminum or an aluminum alloy, due to theirhigh reflectance over a wide spectrum of wavelengths. The mechanicallayer 92 may comprise a metal such as Ni and Cr, and is preferablyformed such that the mechanical layer 92 contains residual tensilestress. The residual tensile stress provides mechanical force tending topull movable layer 66 away from the optical stack 16 when the modulatoris unactuated, or “relaxed.” For convenience, the combination of thehighly reflective layer 90 and mechanical layer 92 is collectivelyreferred to as the movable layer 66, although it will be understood thatthe term movable layer, as used herein, also encompasses a partiallyseparated mechanical and reflective layer, such as the mechanical layer34 and the movable reflective layer 14 of FIG. 7C.

In an embodiment in which the sacrificial layer is to be etched or“released” by a XeF₂ etch, both the reflective layer 90 and themechanical layer 92 are preferably resistant to XeF₂ etching. If eitherof these layers is not resistant, an etch stop layer may be used toprotect the non-resistant layer surface exposed to the release etch.

In an alternate embodiment, the movable layer 66 may be a single layerwhich is both highly reflective and has the desired mechanicalcharacteristics. However, the deposition of two distinct layers permitsthe selection of a highly reflective material, which might otherwise beunsuitable if used as the sole material in a movable layer 66, andsimilarly allows selection of a suitable mechanical layer without regardto its reflective properties. In yet further embodiments, the movablelayer may comprise a reflective sublayer which is largely detached fromthe mechanical layer, such that the reflective layer may be translatedvertically without bending (See, e.g., FIGS. 7C-7E and attendantdescription). One method of forming such an embodiment comprises thedeposition of a reflective layer over the sacrificial layer, which isthen patterned to form individual mirrors. A second layer of sacrificialmaterial is then deposited over the reflective layer and patterned topermit the connections to be made through the second sacrificial layerbetween the subsequently deposited mechanical sublayer and the mirrors,as well as to form apertures in the first sacrificial layer for supportstructures.

In other embodiments in which the MEMS devices being formed comprisenon-optical MEMS devices (e.g., a MEMS switch), it will be understoodthat the movable layer 66 need not comprise a reflective material. Forinstance, in embodiments in which MEMS devices such as MEMS switches arebeing formed comprising the support structures discussed herein, theunderside of the movable layer 66 need not be reflective, and mayadvantageously be a single layer, selected solely on the basis of itselectrical and mechanical properties or other desirable properties.

Finally, in FIG. 9I, it can be seen that a release etch is performed toremove the sacrificial layer, creating the interferometric gap 19through which the movable layer 66 can move. In certain embodiments, aXeF₂ etch is used to remove the sacrificial layer 82. Because XeF₂etches the preferred sacrificial materials well, and is extremelyselective relative to other materials used in the processes discussedabove, the use of a XeF₂ etch advantageously permits the removal of thesacrificial material with very little effect on the surroundingstructures.

Thus, FIG. 9I depicts a portion of an interferometric modulator elementsuch as one of the interferometric modulator elements 60 of FIG. 8,shown along line 9I-9I. In this embodiment, the movable layer 66 issupported throughout the gap 19 by support structures 112 formed overthe movable layer 66. As discussed above, portions of the underlyingoptical stack 16 have advantageously been etched so as to minimize riskof shorting between conductive portions of the optical stack 16 andconductive layers in the movable layer 66, although this step need notbe performed in all embodiments.

FIG. 10 depicts an alternate inorganic post 122 formed from spin-onmaterial, in which a patterning and etching process, rather than ablanket etching back process, is used to form the support structure, andin which some of the spin-on layer overlying the sacrificial layer isnot removed, such that the inorganic post 122 comprises “wings” 124extending out over the sacrificial material 82. Because the layersdeposited over the inorganic posts 122 are deposited over an unevensurface, the edges of these “wing” sections 124 are preferably taperedin order to facilitate the deposition of the additional layers. Whilenot illustrated, it will be understood that a movable layer issubsequently deposited over the inorganic post 122 and sacrificialmaterial 82 of FIG. 10.

In another embodiment, a method for fabricating a MEMS device having anelectrode which is partially detached from an overhanging mechanicallayer is described with respect to FIGS. 11A-11F. This method includesthe steps of FIGS. 9A-9D. In FIG. 11A it can be seen that a reflectivelayer 90 has been deposited over the sacrificial layer 82. In FIG. 11B,the reflective layer 90 of FIG. 11A has been patterned and etched toform isolated electrode member 190 (e.g., isolated mirrors). In FIG.11C, an upper sacrificial layer 182 has been deposited over thepatterned isolated electrode member, and both the upper sacrificiallayer 182 and the lower sacrificial layer 82 are patterned to formapertures 196 extending through both sacrificial layers. As illustrated,these apertures 196 may comprise substantially vertical sidewalls.

In FIG. 11D, support structures 200 comprising a self-planarizingmaterial have been formed within the apertures 196. These supportstructures may be formed, for example, via the process described withrespect to FIGS. 9F and 9G, wherein a layer of self-planarizing materialis deposited over the patterned upper sacrificial layer 182 and thenetched back or patterned to form the support structures 200. It can alsobe seen that a portion of the upper sacrificial layer 182 has beenetched to form an aperture 202 exposing a portion of the isolatedelectrode member 190. In an alternate embodiment, the aperture 202 maybe formed at the same time as the apertures 196, and any insulatingmaterial (e.g., the support structure material) deposited within theaperture 202 in an intervening step can be removed.

In FIG. 11E, it can be seen that a mechanical layer 92 has beendeposited over the patterned upper sacrificial layer 182 and the supportstructures 200, such that the mechanical layer 90 fills a portion of theaperture 202, forming a connector portion 204 which provides mechanicalsupport and electrical connection to the isolated electrode member 190.In FIG. 11F, the mechanical layer 90 has been patterned to form desiredstructures and the sacrificial layers 182 and 82 have been removed by arelease etch, forming a MEMS device (e.g., an interferometric modulatorin which an isolated electrode member 190 is spaced apart from theoptical stack 16 by an air gap 19, and wherein a movable layer 66includes a mechanical layer 92 and the isolated electrode member 190which is partially detached from the mechanical layer.

In another embodiment, depicted with respect to FIGS. 12A-12D, it can beseen that a planarizing material can be used to form a structure whichprovides support for a deformable reflective layer both from above andbelow the mechanical layer. This process includes the steps of FIGS.9A-9D. In FIG. 12A, it can be seen that a movable layer 66, which incertain embodiments comprises a reflective layer and a mechanical layer(see FIG. 9H), has been deposited over the unpatterned sacrificial layer82. The movable layer 66 has been patterned to form an aperture 130extending through a portion of the movable layer 66 and exposing theunderlying sacrificial layer 82.

In FIG. 12B, it can be seen that the portions of the sacrificial layer82 underlying the aperture 130 has been etched away, forming a cavity132, and that this etch extends laterally into the sacrificial layer 82near the apertures 130, such that the cavity 132 undercuts a portion ofthe layer 66. The skilled artisan will appreciate than an isotropicetch, selective against the mechanical layer 130 can accomplish suchlateral recessing, although other suitable methods may also be used. Itcan also be seen that through the use of this etching process, asidewall having a reentrant profile is formed, in that the width of thecavity 132 is narrower at a point immediately beneath the movable layer66 than it is at a point lower in the cavity 132.

In FIG. 12C, a layer 134 of spin-on material has been deposited, suchthat it flows through the apertures 130 to fill the cavities 132, andalso extends over the movable layer 66. Other self-planarizing materialscan also be used, as discussed with respect to FIG. 9A-9I. It can beseen that the spin-on material flows to conform to the shape of thecavity 132, such that the spin-on material assumes a convex shape withinthe cavity 132 corresponding to the concave profile formed by the etchwhich creates the cavity 132. Other deposition methods employing liquidprecursors (e.g., electroless or electroplating) can similarly fill thecavity 132 despite the overhanging movable layer 66 and the re-entrantprofile. The deposition of the spin-on material may comprise any of themethods discussed above, including exposure of the partially fabricatedMEMS device to a liquid precursor.

In FIG. 12D, the spin-on layer 134 is patterned and etched to remove thespin-on material located away from the apertures 130, leaving a supportstructure 136 extending both over and underneath portions of the movablelayer 66, partially enclosing the edges of the movable layer 66. Inlater steps, as discussed above with respect to FIG. 9J, a release etchmay be performed in order to remove the sacrificial layer 82.Advantageously, this embodiment provides improved adhesion between thesupport structure and the movable layer 66, as well as a substantiallyor completely flat mechanical layer, which permits better control overthe size of the interferometric cavity.

In the illustrated embodiment, it can be seen that the diameter of theupper portion of the support structure 136 is substantially the same asthe diameter of the lower portion of the support structure 136. However,in alternate embodiments, it will be understood that both the size andshapes of the upper and lower portions of the support structure mayvary, and that the upper and lower portions of the support structure 136need not be symmetrical with respect to one another.

In addition to the spin-on materials discussed above, it will beunderstood that the support structure 136 may also comprise a polymericplanarization material such as photoresist. Advantageously, the use of apolymeric planarization material simplifies the fabrication process asthe deposition and exposure of an additional mask layer overlying thesupport structure layer is not necessary. Because the edge of themechanical layer is partially enclosed by the support structure 136,degradation of the support provided by such a polymeric supportstructure over time is not as much of a concern as it is in embodimentsin which a polymeric post structure merely underlies a movable layer. Inan embodiment in which a polymeric post underlies a movable layer,adhesion between the polymeric post and the movable layer may be poor.By providing a support structure having overlying polymeric material inaddition to the underlying polymeric material, the adhesion between thesupport structure and the movable layer is greatly improved, leading tobetter control of the size of an air gap over prolonged periods of time.

In a variation of the above method, an underlying support structure maybe formed through, for example, the application of a liquid precurspor(e.g., spin-on deposition) to form a layer which fills the cavity 132 ofFIG. 12B but does not extend above the movable layer, or through the useof a blanket etch to etch back the portions of the spin-on layerextending over the movable layer such that the upper surface of thespin-on support structure is at or below the upper surface of themovable layer.

FIG. 13 depicts a stage in a further embodiment of a method for formingsupport structures from self-planarizing material. In FIG. 13, afterperforming the steps of FIGS. 12A-12C, in place of the step of FIG. 12D,it can be seen that at the same time that the spin-on layer 134 ispatterned to form the support structures 136, a portion of the spin-onlayer 134 extending through the aperture 130 is removed, forming anaperture 138 extending through the support structure 136. In embodimentsin which the support structure comprises a sufficiently rigid material,the support structure 136 can be bifurcated, forming two supportstructures. In such an embodment, the support structures may comprise aconductive self-planarizing material, and still support two electricallyisolated portions of the movable layer without shorting between the twoisolated portions as may be desirable for posts or rails at the edges ofupper electrode strips.

In other embodiments, electroplating can be used to form supportstructures which may have substantially vertical sidewalls. In oneembodiment, described with respect to FIGS. 14A-14B, a seed layerdeposited prior to the deposition and patterning of the sacrificiallayer, and the sacrificial layer is used as a mask during anelectroplating process. In FIG. 14A, it can be seen that a metallic seedlayer 140 has been deposited over the optical stack 16, and that asacrificial layer 82 has been deposited over the seed layer 140 andpatterned to form an aperture 142 extending through the sacrificiallayer 82 and exposing a portion of seed layer 140. In the illustratedembodiment, the apertures 142 have substantially vertical sidewalls,although it will be understood that the shape of the aperture will bedetermined at least in part by the etching process.

In FIG. 14B, it can be seen that a plating process, such as anelectroplating process, has been used to form a support structure 144within the aperture 142, the support structure taking the shape of theaperture 144. In such an embodiment, it will be understood that thesacrificial material is preferably an insulating material which will notbe plated during the electroplating process or is protected by anadditional layer (not shown) in order to avoid being plated. Fabricationof the support structure may continue as discussed with respect to otherembodiments, above, forming a MEMS device having a movable layersupported by an underlying post which may have substantially verticalsidewalls, as depicted.

In a further embodiment, shown in FIG. 19, it can be seen that the seedlayer 140 has been patterned prior to deposition of the sacrificiallayer 82, such that the metallic seed layer 140 underlies only theportions of the sacrificial layer 82 surrounding the support structure144, which is formed by a selective plating process. Although theunderlying seed layer may comprise isolated sections of the seed layer140, as illustrated, it will be understood that electroplating may beutilized when the sacrificial layer 82 comprises a conductive material,such as molybdenum, tantalum, or doped silicon. Through proper selectionof the seed layer 140 and the sacrificial layer 82, plating can becontrolled to only occur on the seed layer, and not on the conductivesacrificial layer 82. In one exemplary embodiment, the sacrificial layer82 comprises tantalum, and the metallic seed layer 140 comprises copper.

FIGS. 20A-20C depict an alternate plating process for forming supportstructures having desired shapes. In FIG. 20A, it can be seen that aninsulating sacrificial layer 82 has been deposited over an optical stack16 and patterned to form apertures 142 which may have substantiallyvertical sidewalls, as depicted, and that a seed layer 140 is depositedover the patterned sacrificial layer 82. In one embodiment, the seedlayer may cover the sidewalls of the aperture 142, as shown in theillustrated embodiment.

In FIG. 20B, the portions of the seed layer 140 located away from theaperture 142 are removed, such that the seed layer 140 coats theinterior surfaces of the aperture 142. In FIG. 20C, a plating process isused to form a support structure 146 within the aperture 142. Becausethe seed layer 140 is only located within the aperture 142, the supportstructure 146 will not extend beyond the edges of the aperture 142.Because isolated portions of the seed layer 140 are being plated, anelectroplating process can be used when the sacrificial layer 82comprises a conductive material, and when the seed layer 140 can beselectively plated with respect to the sacrificial material 82.Fabrication of the support structure may continue as discussed withrespect to other embodiments, above, including the formation of amovable layer supported by support structures 146.

In another embodiment, a directional etch may be used to form spacerstructures which provide support for a mechanical layer or deformablereflective layer. FIGS. 15A-15C depict a method for forming such spacersupports. In FIG. 15A, it can be seen that a layer of sacrificialmaterial 82 has been deposited over an optical stack 16 and etched toform apertures 150. In the present embodiment, it can be seen that theaperture 150 may comprise substantially vertical sidewalls. A conformallayer of support material 152 is deposited over the patternedsacrificial layer. In certain embodiments, this layer 152 may comprisean insulating material, such as SiN_(x) or SiO₂, but a wide variety ofsupport materials may be suitable.

In FIG. 15B, it can be seen that the support material 152 has beenanisotropically etched downward, so as to preferentially remove thehorizontal portions of support material 152, but leaving a portion ofthe vertical portions remaining to form spacers 154 (which in theillustrated embodiment may be a single annular spacer located along thesidewall of each aperture 150). In one embodiment, a reactive ion etchmay be utilized to achieve the desired anisotropic etching, but otheranisotropic etching methods (e.g., sputter etching) may alternately beutilized. The spacers formed by the spacer etch have a rounded or slopedinterior surface while the outer surface is substantially vertical, andthus define tapered or narrowed width of the spacers at their upperregions.

In FIG. 15C, a movable layer 66, which in certain embodiments maycomprise a reflective layer 90 and a mechanical layer 92, is thendeposited over the patterned sacrificial layer 82, such that the spacer154 serves as a support structure underneath the movable layer 66. Themovable layer 66 is conformal over the spacer 154, and in particulardirectly over the tapered interior surface of the spacer 154 and toexposed portions of the optical stack 16. The portion of the movablelayer in the aperture 150 is more easily deposited over the slopedspacer 154, due to the tapered interior surface of the spacer 154.Advantageously, because the aperture 150 need not be tapered, due to thesloped spacer 154, it can be seen that the edge of the spacer 342comprises a substantially vertical edge. Since the support structuredoes not comprise an overhang, the likelihood that actuation of themovable layer 66 will cause the edge of the post to flex downward isgreatly reduced. In addition, because the support structure 154 has asubstantially vertical outer surface, the usable area of the device maybe larger than had the support structure included a tapered outersurface or an overhang.

In MEMS devices such as interferometric modulators, in which the size ofthe air gap between an electrode layer and a movable layer affects thecolor reflected by the device in a relaxed position, it is desirable toensure that the movable layer does not flex upward beyond a desiredposition. FIGS. 16A-16C depict a method for fabricating such astructure, which in the illustrated embodiment includes the steps ofFIGS. 9A-9H, but which in other embodiments may include any suitablemethod for forming a movable layer supported by underlying supportstructures.

In FIG. 16A, it can be seen that an upper layer of sacrificial material182 is deposited over the movable layer 66, and patterned to formapertures 186, which overlie at least some of the underlying poststructures 112. The post structures 112 can thus provide support for theoverlying support structures which will be formed, and so that theoverlying post structures do not overlie the active areas of thedisplay. It will be understood that, in embodiments employing hangingelectrodes (e.g., mirrors) below a mechanical layer, the uppersacrificial layer can be the third sacrificial layer in the device, andan intermediate sacrificial layer may be used to space a portion of thehanging electrode apart from a mechanical layer. In the illustratedembodiment, the apertures 186 are depicted as having substantiallyvertical sidewalls, although apertures having different shapes may beused, examples of which are discussed below with respect to FIGS.17A-17B. The thickness of the deposited upper sacrificial layer 182 mayvary based on the operating parameters of the MEMS device, but incertain embodiments, the thickness of the deposited upper sacrificiallayer is preferably between 30 and 500 angstroms, and more preferablybetween 50 and 200 angstroms, in order to minimize expansion of theunderlying optical cavity being formed, although it will be understoodthat thicknesses both inside and outside of that range may be suitablefor a given application and may be used.

In FIG. 16B, it can be seen that a support structure 162 overlying themovable layer 66 has been formed within the aperture 186. The uppersupport structure 162 may be formed by a method such as that discussedwith respect to FIGS. 9F-9G, wherein a layer of self-planarizingmaterial (see layer 110 in FIG. 9F) is deposited over the patternedsacrificial layer 182 and then blanket etched back to form the uppersupport structure 162. A ceiling layer 192 is then deposited over theupper support post 162. The ceiling layer 192 is preferably a rigid,insulating layer, such as an inorganic oxide layer (e.g., a form ofsilicon oxide), so as to prevent deformation of the ceiling layer 192itself by a movable layer 66 pressing upwards against the ceiling layer192, and to prevent shorting between otherwise electrically isolatedportions of the movable layer 66. In certain embodiments, to aidrigidity, the ceiling layer 192 may be roughly 2-5 times, morepreferably about 3 times the thickness of the movable layer 66, althoughit will be understood that the desirable thickness of the ceiling layer192 will vary based on the composition of both the ceiling layer 192 andthe movable layer 66. In order to minimize deflection of the ceilinglayer 192 itself, the ceiling layer may be formed from a single layer,or may be formed from a plurality of symmetrical layers (not shown),such that the upper layers of the ceiling layer are the substantiallysame material and thickness as the lower layers of the ceiling layer,and the ceiling layer 192 is roughly a mirror image about a neutralaxis.

In FIG. 16C, it can be seen that a release etch has been performed toremove the lower sacrificial layer 82 and the upper sacrificial layer182, forming a gap 19 between the movable layer 66 and the electrodelayer within the optical stack 16, as well as an upper gap 119 betweenthe movable layer 66 and the rigid ceiling layer 192. Both the movablelayer 66 and the ceiling layer 192 are supported by a support structurecomprising the upper support structure or segment 162 and the lowersupport structure or segment 112. The height of the upper gap 119 isdependent on the height of the second sacrificial layer 182 (see FIG.16B), and preferably made as small as possible without being so small asto result in undesirable stiction between the movable layer 66 and theceiling layer 192, or otherwise inhibiting the operation of the MEMSdevice. In a further embodiment, the movable layer 66 can be depositedsuch that the movable layer 66 is designed, upon release, to flex upwardagainst the rigid ceiling layer 192 when the device is in a relaxed, orunactuated, position. Such an embodiment ensures that the movable layer66 will remain at a desired distance from the electrode layer when in arelaxed height, providing uniformity both within a MEMS element andacross an array of MEMS elements.

FIGS. 17A-17B illustrate an alternate method of forming a MEMS devicecomprising a ceiling layer, which again includes the steps of FIGS.9A-9H or any suitable alternative methods. In FIG. 17A, it can be seenthat an upper layer of sacrificial material 182 (having a thickness asdescribed with respect to FIG. 16) has been deposited over the movablelayer 66, and patterned to form apertures 188, which in this embodimentcomprise tapered sidewalls. In FIG. 17B, a layer of conformal supportmaterial 172 has been deposited over the patterned upper sacrificiallayer 182, such that support structures are formed from the supportmaterial within support regions 174, and the support material extendsover the substantially flat portions of the sacrificial layer 192 inceiling regions 176. The process may then continue as discussed withrespect to FIG. 16C, wherein a release etch is performed to form gapsbetween the movable layer 66 and both the electrode within the opticalstack 16 and the ceiling regions 176. A support structure is thus formedwhich includes a lower support segment 112 and an upper support segment174 which is a part of the ceiling layer itself. In a further embodiment(not shown), support layer 172 may be patterned to form isolated supportsegments, and a separate ceiling layer may be deposited over theseisolated support segments.

FIG. 18 illustrates another embodiment of a MEMS device comprising aceiling layer. In the illustrated embodiment, a single contiguoussupport structure 180 extends at least through the movable layer 66,enclosing a portion of the movable layer 66. Such an embodiment may beformed, for example, through the deposition of a lower sacrificiallayer, followed by the deposition of a movable layer 66, followed by thedeposition of an upper sacrificial layer. A via or aligned vias can beetched through each of those layers, forming a single cavity extendingthrough all three layers, which can then be filled with aself-planarizing material in which the movable layer 66 is embedded. Ina further embodiment, a via may be formed through the ceiling layer 192,as well, and the support structure 180 may enclose or embed a portion ofthe ceiling layer 192. In addition to the embodiments discussed above,it will be understood that a wide variety of alternate supportstructures and methods of fabricating the same may also be used to spacea rigid ceiling layer apart from the movable layer.

It will be understood that various combinations of the above embodimentsare possible. Various other combinations of the support structuresdiscussed above are contemplated and are within the scope of theinvention. In addition, it will be understood that support structuresformed by any of the methods above may be utilized in combination withother methods of forming support structures, in order to improve therigidity and durability of those support structures, or to minimizedeflection due to stress mismatches.

It will also be recognized that the order of layers and the materialsforming those layers in the above embodiments are merely exemplary.Moreover, in some embodiments, other layers, not shown, may be depositedand processed to form portions of an MEMS device or to form otherstructures on the substrate. In other embodiments, these layers may beformed using alternative deposition, patterning, and etching materialsand processes, may be deposited in a different order, or composed ofdifferent materials, as would be known to one of skill in the art.

It is also to be recognized that, depending on the embodiment, the actsor events of any methods described herein can be performed in othersequences, may be added, merged, or left out altogether (e.g., not allacts or events are necessary for the practice of the methods), unlessthe text specifically and clearly states otherwise.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device of process illustrated may be madeby those skilled in the art without departing from the spirit of theinvention. As will be recognized, the present invention may be embodiedwithin a form that does not provide all of the features and benefits setforth herein, as some features may be used or practiced separately fromothers.

1. A method of fabricating a MEMS device, comprising: providing asubstrate; depositing an electrode layer over the substrate; depositinga sacrificial layer over the electrode layer; depositing a movable layerover the sacrificial layer; patterning the movable layer to form anaperture extending through the movable layer, thereby exposing a portionof the sacrifical layer; etching the exposed portion of the sacrificiallayer to form a cavity extending through the sacrificial layer andundercutting a portion of the movable layer; and depositing a layer ofself-planarizing support material to fill the cavity.
 2. The method ofclaim 1, wherein the layer of self-planarizing material extends over themovable layer, additionally comprising patterning the layer of supportmaterial to form a support structure, wherein the support structurecomprises a convex outer surface corresponding to a re-entrant profileof the cavity.
 3. The method of claim 2 additionally comprisingpatterning the layer of support material, leaving a portion overlying atleast a portion of the aperture in the movable layer and extending overat least a portion of the movable layer.
 4. The method of claim 3,wherein patterning the layer of support material to form a supportstructure additionally comprises removing a portion of the supportmaterial extending though the aperture in the mechanical layer, formingan aperture extending through the support structure.
 5. The method ofclaim 1, additionally comprising perfoming a release etch to remove thesacrificial layer, forming an air gap located between the movable layerand the electrode layer.
 6. The method of claim 1, wherein depositingthe layer of support material comprises exposing the partiallyfabricated MEMS device to a liquid precursor.
 7. The method of claim 6,wherein the liquid precursor comprises a spin-on precursor.
 8. Themethod of claim 1, wherein the layer of support material comprises apolymeric material.
 9. The method of claim 1, wherein etching theexposed portion of the sacrificial layer to form a cavity comprisesperforming an isotropic etch.
 10. The method of claim 1, whereindepositing a movable layer over the support structure comprisesdepositing a reflective sublayer over the support structures anddepositing a mechanical sublayer over the reflective sublayer.
 11. Themethod of claim 1, wherein the MEMS device comprises an interferometricmodulator.
 12. A MEMS device formed by the method of claim
 1. 13. A MEMSdevice, comprising: a substrate; an electrode layer located over thesubstrate; a movable layer located over the electrode layer, wherein themovable layer is generally spaced apart from the electrode layer by anair gap, said movable layer comprising an aperture extending through themovable layer; and a support structure located at least partiallybeneath the aperture in the movable layer, said support structurecomprising a convex sidewall portion located underneath the movablelayer.
 14. The MEMS device of claim 13, wherein the support structurecomprises a self-planarizing material.
 15. The MEMS device of claim 14,wherein the support structure comprises a spin-on material.
 16. The MEMSdevice of claim 14, wherein the support structure comprises a polymericmaterial.
 17. The MEMS device of claim 13, wherein a portion of thesupport structure extends through the aperture in the movable layer, andwherein said support structure encloses at least a portion of themovable layer located adjacent the aperture in the movable layer. 18.The MEMS device of claim 17, wherein the support structure comprises anaperture extending through support structure to an underlying layer, andwherein said aperture extending though the support structure extendsthrough the aperture in the movable layer.
 19. The MEMS device of claim13, wherein the movable layer comprises a mechanical sublayer and areflective sublayer located on the side of the mechanical sublayerfacing the air gap.
 20. The MEMS device of claim 19, additionallycomprising a partially reflective layer located on the opposite side ofthe air gap from the reflective sublayer.
 21. A MEMS device, comprising:first means for electrically conducting; second means for electricallyconducting; and means for supporting said second conducting means oversaid first conducting means, wherein said second conducting means ismovable relative to said first conducting means in response togenerating electrostatic potential between said first and secondconducting means, and wherein said supporting means extend through anaperture in said second conducting means and enclose at least a portionof said second conducting means.
 22. The MEMS device of claim 21,wherein the first conducting means comprises an electrode layersupported by a substrate.
 23. The MEMS device of claim 21, wherein thesecond conducting means comprises a movable layer, portions of which arespaced apart from said first conducting means by an interferometric gap.24. The MEMS device of claim 21, wherein the supporting means comprisesa support structure which extends through an aperture in the secondconducting means and encloses at least a portion of the secondconducting means, wherein the support structure is formed from aself-planarizing material.